The recent discovery of room-temperature superconductivity does not represent the end of research in superconductivity. Starting with the first discovery of superconductivity in 1911, the study of superconducting properties progressed from phenomenology to the microscopic perspective BCS theory. However, with the discovery of a series of unconventional superconductors, the progress of related theories encountered significant difficulties. A possible path to understanding unconventional superconductivity is to distinguish three-dimensional, (quasi-)two-dimensional, and (quasi-) one-dimensional superconductors to investigate the variation of superconducting properties as a function of dimensionality. In this thesis, we study a series of superconductors in different dimensionalities to unc...[ Read more ]

The recent discovery of room-temperature superconductivity does not represent the end of research in superconductivity. Starting with the first discovery of superconductivity in 1911, the study of superconducting properties progressed from phenomenology to the microscopic perspective BCS theory. However, with the discovery of a series of unconventional superconductors, the progress of related theories encountered significant difficulties. A possible path to understanding unconventional superconductivity is to distinguish three-dimensional, (quasi-)two-dimensional, and (quasi-) one-dimensional superconductors to investigate the variation of superconducting properties as a function of dimensionality. In this thesis, we study a series of superconductors in different dimensionalities to uncover the properties of unconventional superconductors.

In Chapter 3, we investigate the interfacial superconductivity generated by the proximity effect from a superconducting niobium layer deposited onto a quantum anomalous Hall insulator (QAHI) (Cr_0.12Bi_0.26Sb_0.62)₂Te₃ through point contact spectroscopy measurements. By varying the magnitude and sign of the applied magnetic field in the perpendicular direction to the film, we can study the proximity-induced superconductivity when the QAHI magnetization is reversed. According to the theoretical prediction, the superconductivity formed in the QAHI boundary state is a chiral topological superconducting state, and its quasiparticles are chiral Majorana modes. Due to the local nature of the point-contact measurements, we probe the superconducting spectra on a length scale smaller than the size of magnetic domains, thus avoiding the effects of a chaotic and stochastically distributed nature. The topological properties of the QAHI/Nb heterostructures change abruptly during the magnetization reversal when the Chern number changes from N=±2 (represented by the state with two chiral Majorana modes) to N=±1 (one single Majorana mode) and passing through a trivial insulating state with N=0. Our measurements on three devices indicate this series of topological transitions, which agree with the theoretical predictions of a dip-like feature in the point-contact spectra for the N=±2 state and a plateau-like feature for N=±1 and represents the existence of chiral Majorana modes.

In Chapter 4, we investigate the helical conductance in quasi-1D nanoribbons of the same (Cr_0.12Bi_0.26Sb_0.62)₂Te₃ quantum anomalous Hall insulator (QAHI). We fabricate the nanoribbons with a width between 75 and 100nm with the help of gallium focused ion beam system. When shrinking the width of the quantum anomalous Hall insulator, the chiral edge modes from opposite sides should hybridize at a critical width of ~ 100nm, leading to the opening of a hybridization gap and a single helical conduction channel across the nanoribbon. Our magnetotransport experiment demonstrates that the chiral edge mode of the QAHI can be channeled through such narrow nanoribbons without significant dissipation, thus suggesting that a helical transport channel is indeed formed.

Chapter 5 investigates the superconducting gap symmetry of NbSe₂ from monolayer to few-layer thickness by applying a magnetic field, which is rotated in the plane. 2D NbSe₂ exhibits a special type of Ising spin-orbit coupling, which makes firmly pins the electron spins to the out-of-plane direction. The Ising SOC causes a so-called Ising superconducting state, which helps the superconductor withstand very high in-plane magnetic fields that far exceed the Pauli limit for superconductivity. Detailed measurements were performed on several samples, and it was found that for the monolayers, a sixfold nodal symmetry of the upper critical field appears, which agrees perfectly with the theoretical prediction of a nodal topological superconducting phase with six pairs of point nodes that should exist near the upper critical field in high parallel fields. Surprisingly, at lower fields, the resistivity exhibits a twofold in-plane symmetry, which is at odds with the trifold crystalline symmetry, similar to the nematic order previously observed in Nb-doped Bi₂Se₃. We attribute the latter’s origin to the presence of several competing superconducting channels.

Chapter 6 extends our investigation to bulk transition metal dichalcogenide superconductors in high parallel magnetic fields to investigate whether Ising superconductivity or other unusual superconducting states can also exist in bulk. By strictly controlling the alignment of the magnetic field parallel to the layered structure while measuring magnetic torque, specific heat, and thermal expansion, we find that the upper critical magnetic field in the NbS₂ bulk single crystals (a material similar to NbSe₂) also exceeds the Pauli limit. The upper critical field first shows a saturation near the Pauli limit of 10 T in the H-T phase diagram, followed by a pronounced upturn at lower temperatures. In addition, a phase transition anomaly is observed near 10T within the superconducting state. These observations agree with the formation of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Our theoretical calculations show that Ising superconductivity does not play a major role. In the bulk case, the Fulde- Ferrell-Larkin-Ovchinnikov state is responsible for the superconductivity in magnetic fields above the Pauli limit for superconductivity.

Finally, by studying thinner samples of NbS₂, thus inducing the crossover to a 2D material, we investigate the effect of anisotropy as a function of thickness. We prepared and studied exfoliated NbS₂ thin films with thicknesses in the crossover region between 2D materials and the bulk. Combining in-plane and out-of-plane angular-dependent electric magnetotransport measurements, we find that the materials show quasi-2D superconducting properties, possibly due to the weak coupling between the layers.